The present invention relates in general to an RF circuit support architecture for securely retaining and providing for convective cooling of multiple printed circuit cards containing RF signaling circuits and components. In particular, the present invention is directed to a support structure in which RF printed circuit board support and cooling modules are effectively xe2x80x98stackedxe2x80x99 generally transverse to a first side of a base plate in mutually adjacent, spatially separated relationship. Each module includes a convectively cooled heat exchanger to which the RF printed circuit card is mounted and which extends into a gap between mutually adjacent boards. RF signal distribution networks associated with the RF printed circuit boards are disposed on the opposite side of the base plate, and are electrically (RF). coupled to the RF printed circuit boards of the modules by means of square post type multi pin connectors that extend through the base plate.
A variety of communication systems are designed to be environmentally robust in terms of their hardware and signaling format. As a non-limiting example, for the case of a communication system intended for use with a plurality of UHF line-of-sight and satellite links, a multi-link transceiver mounting rack may contain diverse pieces of communication equipment, that typically include RF transmitters, RF receivers, and various digital signal processing units, which control the operation of the RF signaling components, and interface digital communications signals with attendant signal processing circuits. Since each communication link has its own dedicated signalling scheme (modulation format, link protocol, band occupancy assignment, etc.), suppliers of such equipment will typically provide each system as an integrated unit.
In a conventional multi-transistor RF power amplifier, it has been customary practice to mount the power divider, power combiner, power transistors, and associated circuitry, all on the same plane. This xe2x80x98co-planarxe2x80x99 housing approach has several drawbacks. First, not only is considerable area required for the entire assembly, but if a divider/combiner is employed, it becomes a significant portion of the overall layout, as the length of the transmission lines is dictated by the frequency of operation. A divider/combiner is preferred over commercially available surface mount couplers because it has less power loss.
Although they are relatively small sized devices, surface mount couplers require long lengths of transmission line between couplers and active circuitry, as the spacing of the active devices dictates the size of the assembly and is greater than the size of the coupler. These long lengths of transmission line dissipate power. The dissipated loss of each surface mount coupler is also greater than that of a microstrip hybrid coupler installed on a high quality dielectric. Microstrip hybrid couplers can be designed os that they occupy most of the space between the active devices and inter-connecting transmission line length is minimized. Surface mount couplers offer only a two-way power divider/combiner, so that the amplifier must have a binary number of combined stages. Properly designed cascaded hybrids can be employed to combine any number of stages from 2 to N.
In addition, complexity is added to the assembly by the necessity of physically separating the power transistors and the divider/combiner (usually by means of dividing walls), in order to ensure electrical isolation. Moreover, when more than two active stages are combined, it is a challenge to deliver DC power to the center devices without compromising the RF transmission paths. One solution would be to run the DC distribution beneath the RF paths; however, this method increases the cost and complexity of the assembly. Also, it is difficult to match the performance of all of the active devices, both in gain and phase, without some provision for subdividing the assembly into smaller sub-circuits for testing. Each port of the divider/combiner and the active amplifier sections can be connectorized to allow for ease of testing and matching. This solution increases costs, due to the number of connectors required, and increases size due to the length of the connectors.
Pursuant to the invention, the above-described shortcomings of a conventional RF circuit housing architecture are effectively obviated by means of an xe2x80x98orthogonally stackedxe2x80x99 support assembly, that is configured to provide maximum power density for a given volume, while still being able to dissipate heat generated from RF components, in particular, RF power amplifier transistors. As will be described, the support architecture of the present invention arranges a plurality of RF amplifier circuit cards (each of which may include a driver stage transistor and a pair of push-pull transistors for the output stage) on associated edge-mountable RF printed circuit support and heat dissipation modules.
These edge-mountable modules are affixed to a first side of a base plate, so that the RF circuit cards or boards mounted thereto extend generally in a direction that is essentially transverse or orthogonal to the base plate. Arranged on a second side of the base plate are RF distribution networks for the RF circuit cards, that may include quadrature hybrid couplers for both power divider and complementary power combiners. These RF signal distribution networks are RF-coupled to the RF printed circuit boards by means of relatively inexpensive blind-mating, square post type multi-pin connectors that extend through the base plate.
Thus, in the invention, rather than being placed side-by-side on a common heat sink, the active amplifier sub-circuits are effectively mounted in a xe2x80x98stackedxe2x80x99 modular arrangement and having individual heat sinks. For each module, the area of the transistors and associated circuitry of each RF amplifier circuit card is the same, as is the case with their associated divider/combiners on the second side of the base plate. Since the divider and combiner circuits are oriented in a plane orthogonal to the RF circuit cards, the required area and volume is considerably reduced.
RF and DC connections for a respective RF circuit card are provided at one edge of a module""s RF printed circuit card, with input and output RF connections spaced apart at opposite ends of the RF circuit board and DC power connected near the center of the board. In contrast, a conventional co-planar layout usually places the RF connections at opposite ends of the circuit. Thus, all of the RF amplifier circuit cards may be identical, since there are no special considerations as to the routing of DC power to centrally located cards.
This adjacent parallel mounting or xe2x80x98stackingxe2x80x99 of the RF sub-circuit cards allows each to be individually shielded, so as to ensure very high isolation between each active RF circuit card and the power divider and combiner circuitry on the opposite side of the base plate. Also, within the RF distribution circuitry on the bottom of the base plate, the RF combiner circuitry may be electrically separated from the RF combiner by a center wall region therebetween along which DC power is provided. The three-pin configured post-type RF connectors are designed so that they are completely shielded by conductive material (e.g., aluminum) of the assembly so as to minimize electromagnetic interference (EMI) and RFI leakage. RF power may be supplied at the top of the assembly and RF output derived at the bottom, thereby providing a maximum separation to minimize feedback.
DC power distribution is readily accomplished by mounting a separate circuit board to the back side of the divider/combiner support plate into which each edge-mounted RF circuit card and heat sink assembly is plugged. The design of the divider/combiner on the underside of the base plate is such that RF connections are located near the outer edge of the assembly. The area between the RF connectors, on the back side of the divider/combiner assembly, is then used to support a DC power distribution board. This area would normally be wasted space in a conventional co-planar amplifier layout. However, with the DC distribution board attached to the divider/combiner assembly, all of the RF and DC connector pins are aligned with one another, allowing a card mounting assembly to be readily blind-mated and held in position with screws. A small amount of floating of the square-post three-pin connectors allows for alignment during assembly. DC power is delivered at a common point on the top of the assembly.
Advantageously, with the architecture of the invention, the active device sub-circuits can be individually tested and aligned, to ensure that the power division and re-combination is optimal, before being installed in the assembly. The stacked alignment of the RF circuit card modules ensures that the gain and insertion phase falls within certain established limits, which allows for easy repair of the overall RF amplifier by replacing a failed card with a suitably aligned spare. Conventional co-planar designs may require intensive disassembly, alignment and re-assembly.